Noble gas atoms occur in nature with unique isotopic ratios depending upon the source of the atoms. A wide variety of applications for isotopically selective counting of these noble gas atoms has been identified, and many of such applications necessitate the counting of a small number of atoms of one isotope even when the neighboring isotope may be more abundant by twelve orders of magnitude. Some of the applications are: (1) ground water dating by detecting the small number (e.g., 500) atoms of Kr-81 removed from 1 liter of water; (2) polar ice dating by also detecting small quantities of Kr-81; (3) atmospheric studies through the detection of Kr-85 and isotopes of Xe; and (4) ocean water circulation where a few atoms of Ar-39 are present with as many as 10.sup.19 atoms of Ar-40 in a liter of ocean water.
A variety of measuring techniques have been developed to be utilized in the detection of, in particular, radioactive isotopes. One such technique utilizes radioactive decay. This is a slow process, and is not effective for very small quantities (even down to one atom). It is also impractical for radioactive isotopes having very long half-lives, as is the case for Ar-39 and Kr-81. Furthermore, conventional mass spectrometers are limited in sensitivity, requiring more than 10.sup.8 atoms in most cases, and suffer from isobaric interferences. Thus, more rapid and precise techniques have been needed, and some have been developed. Several of these improved techniques are based, at least partly, upon resonance ionization spectroscopy (RIS) as disclosed in U.S. Pat. No. 3,987,302, issued on Oct. 19, 1976, and assigned to the common assignee of this invention. This patent is incorporated herein as reference for any teaching that is not covered in sufficient detail herein.
Two other patents assigned to the common assignee, and incorporated therein by reference for their teachings, are U.S. Pat. No. 4,426,576, issued Jan. 17, 1984, and U.S. Pat. No. 4,442,354, issued Apr. 10, 1984. One of the inventors of all of these cited patents is one of the inventors of the present invention. 0f these later two patents, the '576 patent is most pertinent to the present invention. It specifically is directed toward the detection of noble gas atoms. It provides for a method in which the vacuum is static during the collection of separated atoms, and is followed by active pumping to remove residual materials before a cycle can be repeated. A substantial length of time is required for counting of the desired (selected) atoms. Furthermore, use of a static vacuum system causes interferences due to background atoms. In addition, the method and apparatus of that patent can only accomodate one sample at a time so that a comparison cannot be made with another sample or "standard".
Accordingly, it is a principal object of the present invention to provide a method and an apparatus that permits the counting of a very few atoms of a selected specie wherein there is a very abundant quantity of potentially interferring species, the method providing for very rapid counting.
It is another object of the present invention to provide a method of counting very few atoms of one isotope of an element in the presence of an abundant quantity of atoms of other isotopes of that element, with that method providing rapid information.
It is still another object of the present invention to provide a method of and apparatus for counting very few atoms of one substance in one sample, such as an isotope of an element, in the presence of another substance or isotope of abundant quantity in that sample, and wherein such counting can be accomplished during the counting of similar atoms from a second sample.
These and other objects of the present invention will become apparent upon a consideration of the detailed description and by reference to the drawings.